The present invention relates generally to cloth dust collector processes and apparatuses wherein dust laden gas streams are conducted through cloth or fabric filtration elements, thereby separating the dust on the collection side of said elements while allowing the gas stream, freed of its dust burden, to pass through to the exhaust side and to be thereafter removed and treated in any conventional or desired manner. More particularly, the present invention relates to an improved detection method for such apparatuses.
In many industrial processes there are produced products or byproducts in suspended dust form, that is to say, in the form of solid particulate matter entrained in a gaseous medium. For instance, furnace and thermal carbon blacks manufactured by the thermal decomposition or partial combustion of hydrocarbonaceous feedstocks are normally first produced in the form of a suspension of the carbon black product entrained in a byproduct flue gas stream. The process stream is conducted from the carbon black forming reactor, cooled and then subjected to product separation and collection. Conventionally, said separation and collection of the carbon black product from the flue gas stream is accomplished by bag filtration thereof.
The general bag filtration and collection technique generally comprises the flowing of a dust laden gaseous stream through one or more porous cloth or fabric bag filtration elements, said elements having a porosity which is, at once, sufficient to allow the gas stream to pass therethrough while being insufficient to allow passage of the dust component. As a result, the dust component is separated from the gas stream and is deposited on the collection side of the filter bags. Means are usually provided by which to periodically remove the collected dust from the bags, such as by repressurization or reversal of the gas flow through the bags, mechanical shaking or vibration of the bags and the like. Upon removal from the filter bags, the separated dust is normally conveyed into a collection hopper and is periodically removed therefrom.
The filter bags are usually composed of woven or unwoven textile materials such as glass, cotton, wool, polyamide or polytetrafluoroethylene fibers. Said filter bags are disposed in one or more compartments of an enclosed bag filter vessel and are affixed to an apertured partition or "cell plate" thereacross which, in essence, divides said vessel into a collection side and an exhaust side and, in consort with the affixed bags, isolates the exhaust side from contact with the dust component introduced into the collection side. In operations the dust laden process stream is introduced into the collection side of the vessel whereupon it flows through the filter bags, thereby to deposit the dust component thereof on the collection side thereof. Meanwhile, the gaseous components of the process stream pass through the filter bags into the exhaust side of the vessel whereupon they are exited and treated or otherwise disposed of in any suitable manner. Generally, the process stream and the bag filtration apparatus are maintained at temperatures above the dewpoint of the process stream in order to mitigate against condensation and occlusion of the filter bags by condensed liquids. During operations, as the dust collects on the filter bags, the pressure drop across the bag filter tends to increase. Thus, it is conventional practice that the filter bags be periodically purged of their respective collected dust burdens upon attainment of a preselected maximum allowable pressure drop across the apparatus, such as by mechanical shaking or by reverse gas flow through the bags. In certain commercially available bag filters the filter bag elements are not, strictly speaking, in the generally tubular "bag" form but rather are envelope-shaped. In some bag filters the filter bags are hung, without substantial support, between an upper hanger plate and the apertured cell plate of the vessel. In other bag filters each filter bag or element is provided with a rigid or semi-rigid cage or screen skeletal support structure and which structure is adapted to prevent stretching or collapse of the filter element. Further details relating to bag filter design and operations may be had by reference to Chemical Engineers' Handbook, John H. Perry, Third Edition, 1950, McGraw-Hill Book Company, Inc., pgs. 1029-1034.
During the course of operations of bag filter apparatus there normally ultimately occurs deterioration of one or more of the filter bags to the extent that leaks develop through which dust can escape from the collection side into the exhaust side of the vessel. Leaks can also develop in the isolating components of the bag filter other than the cloth filter bags themselves such as, for instance, by failures or weldments, gaskets, improper filter bag installations and the like. These leaks, of course, are deleterious since they can lower the collection efficiency of the apparatus and can cause great difficulties in the downstream handling or disposal of the dust adulterated or polluted gas stream conducted from the exhaust side of the bag filter vessel. It is, therefore, obviously necessary that such leaks be detected and remedied. In prior art practice, bag filter leak detection has generally been accomplished simply by arduous, detailed and time consuming visual inspection of the passive bag filter after it has been taken off the process line, purged of noxious gaseous components and cooled. Accordingly, the enclosing vessels of bag filters are normally provided with access hatches which allow entry of service and inspection personnel into the collection and exhaust sides thereof. As mentioned, leak detection is conventionally undertaken by personnel who enter the vessel and undertake careful visual inspection of the interior of the bag filter. Over and above the tedious and time consuming nature of the task, such passive visual inspections also often fail to elicit the locations of all significant leak sites with the result that the bag filter, once repaired to the satisfaction of the inspection and service crew, is often discovered to continue to have significant leaks when placed back into service. This, of course, can require that the filter be again taken off the process line, cooled, purged and reinspected. Passive visual inspection of bag filters is often further complicted and made even more difficult by the nature of the process dust collected therein. Thus, bag filters employed in the separation and collection of a dark colored dust, such as carbon black, are particularly difficult to inspect for leaks since the dust residuum in the bag filter tends to mask leak sites and the uniform dull black or grey colorations of the filter bags normally found in such a bag filter mitigates strongly against effective visual inspection thereof. In accordance with the present invention, however, there is provided a method for bag filter leak detection by which the efficiency and effectiveness of leak detection by visual inspection is markedly improved.